The development and use of fuel cells is by many seen as the most promising route to reduce CO2-emissions and increase the efficiency of power production. Selection and distribution of feedstock for small-scale fuel cell units are the main challenges in the commercialisation. Conversion of hydrocarbon feeds ranging from natural gas to gasoline and diesel into hydrogen for the fuel cells is an accomplished technology. Steam reforming of methanol is also well known and very compact units can be constructed. Use of ethanol which can be obtained from biomass is attracting increasing attention both as primary feedstock and as an alternative to increase the feedstock flexibility in a given unit.
Steam reforming of ethanol is not a straight forward process. Ideally, the conversion of ethanol into hydrogen could follow reaction (1):C2H5OH(g)+H2O(g)2CO+4H2 ΔH298=255 kJ/mole  (1)
Equilibrium of reaction (1) is shifted towards the right even at low temperature. However, in practice ethanol is also converted to ethylene according to reaction (2):C2H5OHC2H4+H2O ΔH298=45 kJ/mole  (2)
Ethylene will even in small amounts rapidly form carbon and thereby carbon polymer on most steam reforming catalysts which causes considerable deactivation. High ethylene contents should therefore be avoided.
A dual catalytic system involving the initial conversion of ethanol to ethylene and thereafter to ethane can be used instead and followed by steam reforming of the ethane. In the first catalyst zone, ethanol is converted into ethylene according to reaction (2). Furthermore, ethylene reacts with hydrogen to form ethane according to reaction (3):C2H4+H2C2H6 ΔH298=−137 kJ/mole  (3)
Excess hydrogen suppresses the equilibrium concentration of ethylene. FIG. 1 shows the equilibrium ethylene concentration at various temperatures and pressures for an equimolar feed of ethanol and hydrogen. Low ethylene content is obtained at high pressure and low temperature.
There is therefore the possibility of using hydrogen for suppressing the ethylene as mentioned earlier with respect to reaction (3).
The objective of the invention is to provide an ethanol reforming process whereby the formation of polymer carbon caused by the ethylene intermediate product is suppressed.
Another objective of the present invention is to provide a process, whereby hydrogen needed for hydrogenation of ethylene to ethane is supplied by recycle of an effluent stream obtained during processing of ethanol to hydrogen.